Hammermills are used for grinding or comminuting materials such as corn. Typically, a hammermill consists of a housing having a feed material inlet at the top, a grinding or working chamber below the feed material inlet and a ground material outlet below the working chamber. The working chamber is usually defined by an apertured screen extending downwardly from one edge of the feed material inlet and curving about to form a partly cylindrical surface before extending upwardly to the opposite edge of the inlet. The cross-sectional shape of the chamber is roughly a teardrop formed by a circular lower portion mounted by two straight lines converging upwardly toward the edges of the feed material inlet. The apertured screen defines the walls of the working chamber and surrounds a rotor mounted coaxially in the cylindrical portion of the grinding chamber. A number of hammers are pivotally mounted on the rotor for free-swinging movement under centrifugal force when the rotor is rotated. During rotation, the tips of the hammers pass closely along the surface of the apertured screen, impacting upon the feed materials and hence comminuting the materials until the particles are sufficiently fine to pass through the apertured screen to the particle outlet.
During grinding, however, the particles impacted by the hammers quickly attain the velocity of the hammers tangentially of the screen surface. This is partly due to the impact and partly due to the fanning action of the rotor on the air in the working chamber. Also, the low angle of contact of the particles with the screen prevents passage of even properly sized particles through the apertures so that the particles travel along the screen surface at approximately the same velocity as the hammer tips. This results in a very low number of large energy impacts and an unsatisfactory production rate. Regrind deflectors have been proposed to alleviate this problem, the deflectors being circumferentially spaced about the circular screen to deflect the material inward for higher energy impacts. Such deflectors are disclosed in U.S. Pat. No. 5,503,338. Polygonal-shaped screens have also been proposed to increase the high energy impact of the hammers on the material. See, for example, U.S. Pat. No. 5,692,688, issued Dec. 2, 1997 common assignee herewith.
Further, hammermill screens frequently become damaged by the impact of the particles and through normal wear and tear. The screens must be periodically replaced and, of course, replaced upon the occurrence of a substantial tear in the screen. Screen replacement is oftentimes laborious and time-consuming, and incurs downtime costs for the hammermill. Automatic screen changers have been proposed and constructed but are quite expensive and compromise the structural integrity of the hammermill. Clamp-in screen carriages have also been proposed and constructed but are often difficult to handle due to their size and weight. Additionally, they do not ensure complete edge clamping due to the tangential positions of the clamp.